Further theme work

[crertel]

As per conversation with @boydm , inspired by the good work done by #252 , we've found a way forward that's maybe a little simpler.

(this is loose paraphrasing, we'll collect better notes here as we can recall them)

We want a Scenic.Theme (probably replacing Scenic.Styles.Theme) to encapsulate settings for a given theme. Settings for a theme are things like:

• Colors
• Fonts
• Font-sizes
• Other settings that people might want to customize (tbd, so we'll want the theme container to be flexible in that regard)

A sample theme might be created something like:

hotdog_stand_theme = Theme.create(:hotdog_stand)
                     |> Theme.add_setting( :pallette, %{
                         primary: Scenic.Color.named().red,
                         secondary: Scenic.Color.named().yellow,
                     })
                    |> Theme.add_setting( :font, "comic_sans")

We want to introduce a Scenic.ViewPort.Themes module, which exposes functions for:

• adding a theme at runtime
• removing a theme at runtime
• listing registered themes
• getting a theme at runtime

We probably don't want to do a full genserver (since we can't have everybody hammering that with messages when drawing), but instead we'll probably let folks do concurrent reads on a named ETS table and then marshal mutating operations through the module API (which may touch a genserver or maybe not). This is similar to how Viewport already works, so it'll be helpful to reuse those concepts for developers.

@boydm Lemme know if this is enough to jog your memory. :)

[boydm]

Yes. This is a good direction. Any theme manager is going to have a heavy concurrent read pattern, which means if it needs to query a GenServer, then it is going to serialize. So... Ok to write themes to a GenServer that then writes them to an ETS table, but reads come directly out of ETS on the caller's process.

[vacarsu]

I like the idea of including fonts into themes, but hope you would still be able to swap fonts out without necessarily having to change the theme. The original reason why I didn't go the GenServer/ETS route is because I was under the impression that an ETS lookup was still going to require sending a message to a process and was trying to avoid that.